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Babies born underweight are known to be prone to a variety of cognitive problems.

Babies born weighing less than 4lb (1.8kg) could be more prone to developing autism than children born at normal weight, a study suggests.

Writing in Pediatrics journal, US researchers followed 862 New Jersey children born at a low birthweight from birth to the age of 21.

Some 5% were diagnosed with autism, compared to 1% of the general population.

But experts say more research is needed to confirm and understand the link.

Links between low birthweight and a range of motor and cognitive problems have been well established by previous research.

But the researchers say this is the first study to establish that these children may also have a greater risk of developing autism spectrum disorders.

The babies in the study were born between September 1984 and July 1987 in three counties in New Jersey.

They all weighed between 0.5kg and 2kg or a maximum of about 4.4lb.

At the age of 16, 623 children were screened for risk of an autism spectrum disorder (ASD).

Most low birthweight children don't have autism, and most children with autism don't have low birthweight.Prof Dorothy Bishop, University of Oxford

Of the 117 who were found to be positive in that screening, 70 were assessed again at age 21.

Eleven of that group were found to have an autism spectrum disorder.

From these results, the researchers calculated an estimated prevalence rate of ASD of 31 out of 623 children, which is equal to 5%.

Jennifer Pinto-Martin, professor at the University of Pennsylvania School of Nursing and director of the autism centre where this research was conducted, said: "Cognitive problems in these children may mask underlying autism.

"If there is suspicion of autism or a positive screening test for ASD, parents should seek an evaluation for an ASD. Early intervention improves long-term outcome and can help these children both at school and at home."

Dig deeper

But Dorothy Bishop, professor of developmental neuropsychology at the University of Oxford, said it was important to put the findings in perspective.

"The association looks real, but nevertheless, most low birthweight children don't have autism, and most children with autism don't have low birthweight."

Georgina Gomez, action research leader for The National Autistic Society, said more research is needed to confirm the link between low birthweight and autism and better understand why babies born underweight may be more prone to developing autism.

"Low birthweight has been linked to a range of motor and cognitive problems and often goes hand-in-hand with premature birth and birthing complications.

"It is important to dig down further to try to understand the biological processes and events that could explain this proposed connection."